: Muscle spindles are specialised fibres and they act as receptors to the amount of stretch in a muscle.
: As they become stretched , spindles send impulses via sensory neurones to the spinal cord.
: These synapse with motor neurones that lead back to the muscles (effectors)
: At the muscles these impulses cause the brachialis muscle to contract and other muscles to relax.

: Muscle spindle reflex- one muscle contracts and the other relaxes
: The relaxed muscle is stretched and the muscle spindle is stimulated.